Spread spectrum radio frequency systems are a variety of signal modulation that spreads a signal to be transmitted over a bandwidth that substantially exceeds the data-transfer rate. In direct sequence spread spectrum, a data signal is modulated with a pseudo-random chip sequence. The encoded spread spectrum signal is transmitted to the receiver. The receiver receives the signal and de-spreads the signal. Many techniques are available for transmitters to modulate the data signal, including but not limited to minimum shift keying (MSK).
Conventionally, in de-spreading a spread spectrum signal, a receiver produces a correlation pulse in response to the received spread spectrum signal. When the received spread spectrum signal matches the chip sequence to a predetermined degree, correlation is achieved. Conventionally, a variety of techniques exists for correlating a received signal with a chip sequence, including surface acoustic wave (SAW) correlators, tapped delay line (TDL) correlators, serial correlators, and the like.
A problem which may be encountered in attempting to correlate spread spectrum signals transmitted using MSK techniques, may be the absence of a coherent reference signal in the receiver. A coherent reference signal may be defined as a locally generated signal that matches the transmitter carrier signal in frequency and phase. The receiver may use the locally generated reference signal to demodulate the received signal. In practice, however, it may be difficult to independently generate a local reference signal in the receiver which precisely matches the transmitted carrier signal in frequency and phase. A local reference signal generated in the receiver is often a non-coherent signal which may have differences in frequency and phase from the transmitter's carrier signal. The frequency and phase differences are not constant but vary over time. During an attempt to demodulate a received signal using a non-coherent reference signal, errors in correlation may occur due to mismatches in timing and variations in perceived amplitude caused by the frequency in phase differences.
Various methods for dealing with the above problem exist in which a coherent reference signal is created in the receiver by continuously measuring the frequency and phase differences between the received signal and a locally generated non-coherent reference signal, and then adjusting the non-coherent reference signal until it matches the frequency and phase of the received signal. Such methods, however, generally require the use of a relatively complex feedback technique and require successive amounts of hardware. Further, locking on to the received frequency and phase can take an unacceptably large amount of time, particularly in systems where time is of the essence, such as in certain time division multiple access (TDMA) systems in which only a relatively brief time slot is allocated for periodic communication between a transmitter and receiver.
Non-coherent digital matched filters have been described which use four real filter channels to perform four-phase quantization in the complex plane, with the four quadrants being the quantization regions, and the result taken on the four complex values of ±1±j. In such systems using a four-phase filter, an input signal may be divided into an in-phase signal (I) and a quadrature signal (Q). The in-phase signal and the quadrature signal are separately filtered, sampled and digitized using one-bit quantization. The quantized in-phase signal and the quantized quadrature signal are each fed into two binary correlators each programmed with a reference sequence of N chips, one chip for each sample. The outputs of the four binary correlators are combined to produce a resultant output signal. The described four-phase filter is a system using only one-bit quantization, and is not a technique that may be used for serial correlation.
Hopped MSK waveforms such as Link-16 use two stages to sample and demodulate data. The first step is to detect a synchronization preamble that is used to recover accurate chip timing information. The input sample stream can then be decimated to 1 sample per chip and data is demodulated. Coherent demodulation of the data requires embedding some number of known value chips to act as a phase reference for the rest of the data within the pulse. The remaining chips are then compared to the sum of the phase reference bits to determine whether they are a 1 or a 0.
Accordingly, there is a need for a method of modulation and demodulation particularly suited to MSK signals. Further, there is a need for a system and method of MSK modulation and demodulation that does not require the generation of a coherent reference signal, and that is capable of rapid correlation, and that may be used with analog correlators and digital correlators in an effective manner.
It would be desirable to provide a system and/or method that provides one or more of these or other advantageous features. Other features and advantages will be made apparent from the present specification. The teachings disclosed extend to those embodiments which fall within the scope of the appended claims, regardless of whether they accomplish one or more of the aforementioned needs.